An exemplary radiofrequency transmission device DIS0 is represented by FIG. 1. The device DIS0 is incorporated into an integrated circuit and comprises in a customary manner an antenna ANT linked to receive RX and transmit TX stages on an input-output node I/O, by way of a transformer TR.
The transmit stage TX comprises a power amplifier PA intended to amplify a transmission signal, notably by means of a power transistor M1.
The receive stage RX comprises for its part a low noise amplifier LNA at the output of which a mixer carries out a frequency transposition with a signal generated by a local oscillator LO.
The transmit TX and receive RX stages are naturally implemented in an individual manner, the transmit stage TX being deactivated when the receive stage RX is in operation, and vice versa.
Moreover, in this example the receive stage comprises an attenuator circuit ATNR, making it possible to attenuate the reception signal received on the antenna ANT, so as to decrease the level of the signals picked up by the antenna so as to reduce them to measurable values within the appropriate dynamic range of the receive stage RX.
Customarily, attenuators are formed by circuits incorporated into the receive stage RX on an input-output node I/O situated between the transformer TR and the low noise amplifier LNA. For example, the attenuator ATNR is produced by a network of parallel resistors R1, R2, R3 of various values, wired up in series with respective switches SW1, SW2, SW3 between a ground terminal and the input-output node I/O.
Such an attenuator makes it possible to implement an attenuation of the reception signal according to an adjustable gain, as a function of the needs of the device and of the height of the reception signal.
This standard design introduces large parasitic capacitances, notably because of the switches SW1, SW2, SW3, which compromise the adjustment of the transformer TR and may create distortion of the reception signal.
Low noise amplifiers with degenerate gain make it possible moreover to attenuate a reception signal, but their differential design introduces noise into the receive chain, in particular in low-energy operation.
It is desirable to remedy these defects in common-mode operation, notably within the framework of radiofrequency transmissions of the Wifi or low energy-consumption Bluetooth type “BLE” (the acronym standing for “Bluetooth Low Energy”).